Various methods exist for bonding glass surfaces together. These methods include, for example, wringing, fusion bonding, adhesive bonding and vacuum bonding. Bonding without the assistance of polymeric adhesives is a technology of interest for numerous industries including microelectronics and photonics. Adhesives are sensitive to thermal treatments and can fail from cycling in relatively moderate thermal environments (e.g., 0-200° C.). On the other hand, the formation of a direct bond between two glass or metal surfaces allows for an impermeable seal that has the same inherent physical properties as the bulk material surfaces being bonded. For bonding of dissimilar materials, the resulting bond is sensitive only to CTE mismatches between materials, as compared to polymeric adhesives that typically have a CTE substantially different than at least one of the bulk substrates.
Optical wringing refers to a process of bonding glass surfaces in which adsorbed surface groups are removed from active bonds on a surface by heating the parts to temperatures typically above 600° C. but below the softening point of the glass. Adsorbed water and organics will vaporize and the results surface sites become “active.” At such a temperature or after cooling in a clean, low humidity environment, surfaces can be placed in contact at which point covalent bonds spontaneously form between “active” bonds on each surface. This is similar to vacuum bonding, except the surface is activated by temperature rather than by a strong vacuum. A disadvantage of this process is the effect of high temperatures on polymers that may be associated with the glass article to be bonded, for example, fiber coatings and fiber array systems that utilize polymeric adhesives to bond the fiber array together.
Vacuum bonding involves bringing two clean surfaces into contact in a high vacuum, thus forming a bond. Provided that the surfaces are flat and clean, a high vacuum removes adsorbed water and hydrocarbons from the surface while preventing the adsorption of such species. Surfaces can be cleaved in the vacuum, processed and cleaned before being placed in the vacuum, or cleaned in the vacuum via ion milling or other plasma techniques. One disadvantage of this process is the effect of a high vacuum on polymers that may be associated with the glass article to be bonded, for example, fiber coatings and fiber array systems that utilize polymeric adhesives to bond the fiber array together. High vacuum pressure may have a negative effect on these polymers.
Within the microelectronics field, vacuum bonding has been developed for sealing of such materials as single crystal silicon, thermal oxide SiO2 grown on Si, and various metals, as described in U.S. Pat. No. 6,153,495. Coefficient of thermal expansions (CTE) mismatch between materials is not an issue because the process can be applied at room temperature.
Fusion bonding refers to the process of cleaning two surfaces (glass or metal), bringing the surfaces into contact, and heating close to the softening point of the materials being bonded (to the lower softening temperature for two dissimilar materials), thus forming a welded interface. One example of a fusion bonding process is fusion splicing of optical fibers. Advantages of fusion bonding include the fact that commercial systems exist for splicing of fibers and that the process is relatively easy to apply to bulk geometries. One disadvantage of fusion bonding is that this process typically results in deformation of the two surfaces being bonded due to the flow of softened material, the inability to use this process for complex geometries where adhesives or other low-temperature materials are used, and loss of signal transmitted through the interface when fusion bonding is used for signal transmitting objects such as optical fibers. Furthermore, for bonding of large surfaces, it is difficult to limit glass softening to the bonding interface. As a result, the entire seal can lose dimensional tolerances. In addition, the high temperature ranges required to fusion bond many glass materials are disadvantageous for complex systems that include the use of low-temperature materials such as adhesives and polymer coatings (e.g., fiber coatings).
Adhesive bonding is a common process for mounting of fibers in ferules and for bonding of photonic components such as filters, polarizers, rotators, etc. to each other and into packages. Some advantages of using such adhesive are that they are readily available, UV curable, and allow for alignment of components between application of the adhesive and curing into permanent position. Disadvantages of adhesive bonding include CTE mismatch especially for low CTE materials such as high purity fused silica, for applications where the bonded part is exposed to thermal cycling. Another issue is signal loss from transmission through the adhesive when the adhesive is used in the optical path of optical systems. Although it is possible to utilize an index matching adhesive that has a refractive index matching the optical component, it is extremely difficult to utilize an adhesive that has a CTE and refractive index that matches the optical components. In addition, there are concerns over long-term reliability of packages that incorporate adhesives. Furthermore, bonding of components with adhesives can require angle polishing (typically 8°) and associated assembly to prevent back-reflection.
Another type of bonding process involves chemical bonding. The formation of a chemical bond between two glass or metal surfaces allows for an impermeable seal that has the same inherent physical properties as the bulk material being bonded. In literature, low-temperature bonding technology has been reported for bonding soda-lime-silicate glass and for crystalline quartz (see, e.g., A. Sayah, D. Solignac, T. Cueni, “Development of novel low temperature bonding technologies for microchip chemical analysis applications,” Sensors and Actuators, 84 (2000) pp. 103-108 and P. Rangsten, O. Vallin, K. Hermansson, Y. Backlund, “Quartz-to-Quartz Direct bonding,” J. Electrochemical Society, V. 146, N. 3, pp. 1104-1105, 1999). Both the Sayah and Rangsten references disclose using acid to contact the bonding surfaces. Another article, H. Nakanishi, T. Nishimoto, M. Kani, T. Saitoh, R. Nakamura, T. Yoshida, S. Shoji, “Condition Optimization, Reliability Evaluation of SiO2—SiO2 HF Bonding and Its Application for UV Detection Micro Flow Cell,” Sensors and Actuators, V. 83, pp. 136-141, 2000, discloses low-temperature bonding of fused SiO2 by first contacting the bonding surfaces with hydrofluoric acid. While these bonding processes are useful in certain applications, the bond strength provided by contacting with acidic solutions is limited and could be improved.
It would be desirable to provide a bonding process that does not have the disadvantages of fusion bonding, adhesive bonding, and wringing, and offers more reliable seal integrity than low pH chemical bonding. In addition, it would be useful to provide a bonding process that was durable, provided high bond strength and could be used on a wide variety of silicon-containing materials and surfaces.